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Does the autopilot attempt to recover from a mechanical failure of a single control surface?
I'm asking this question having watched a documentary about Northwest Flight 85 in 2002, during which a 747-400 experienced a lower rudder hardover at an angle of 17 degrees to the left.
The pilots were able to recover by taking over manual control and using the ailerons, the upper rudder and asymmetrical engine thrust to oppose the forces being generated from the lower rudder hardover.
During the doco, I believe the pilot in control immediately turned off the autopilot as he realised it wasn't going to be able to recover from the initial roll.
I just wondered if an autopilot would be able to recover from such an event - i.e. is it technically possible? It seems like the sort of thing an autopilot could achieve easier than a pilot, assuming it was programmed correctly.

Normal autopilots don't. They expect all systems to work correctly, and in case of  failure of a single control surface they will stubbornly command actions which are no longer possible. For them, thrust is only for controlling the vertical speed, and the application of asymmetric thrust for yaw control is not possible.
There are experimental autopilots which compare the result of their actions with a number of pre-programmed scenarios and select for their next commands the scenario which agrees best with reality. These really do what you assumed, but the specific scenario must have been considered before so the specific control laws are part of the code. Computers small enough to be carried on airplanes became powerful enough to do this in real time maybe 20 to 25 years ago, but I am not aware that regular autopilots of airliners have this capability even today. It is, however, used for unmanned vehicles.

I would add that in most planes the autopilot only commands elevators and ailerons and separate yaw damper maintains coordination using the rudder and is usually limited so it can't correct big deviation like this. It keeps the systems simple, but in case of failure the interaction may cause undesired results. In this case the autopilot would counter rudder with ailerons inducing large forward slip and loss of airspeed as there would not be enough thrust for that at cruise level.

military autopilots will be able to recover losing a flight surface, the fly by wire system in modern fighters is designed to do this even in manual flight

Not all autopilots have this capability, but it usually specifically added for airplanes that have split rudders and are highly susceptible to Dutch rolls without active yaw damping (T-tailed, large sweep, significant dihedral). This should be supplemented with adequate indication of the situation to the pilot, and newer glass cockpits are even introducing displays of corrective actions and restricted flight envelopes, where the pilot can have a reduced workload and airframe stress will be minimized.
That said, hardovers are probably the most difficult potential error to design against, and in some cases would be catastrophic if they occurred, regardless of any attempts at correction. If this is the case, it is part of the aircraft manufacturer's job to prove that the probability of such hardovers is statistically impossible (usually defined as less than one expected occurrence per billion hours of flight).

If you have an F15 you don't need wings...


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Where can I find a flight log archive for commercial flights?
I heard that commercial flight logs are publicly available from the FAA, including mechanical delay reasons, etc.  Is this true?  If so, where can this information be found?

Under the Freedom of Information Act you can request these records from the FAA. Please read more about doing so here:
http://www.faa.gov/foia/foia_request/

The data you want is here: http://www.transtats.bts.gov/DL_SelectFields.asp?Table_ID=236&DB_Short_Name=On-Time
Select pre-zipped file to get all fields. You have to download it one month at a time.

I believe Flight Aware allows you to view filed flight plans. It also lets you know where the aircraft is in-flight, although I am not sure how accurate that information is.

Yes, I did find my flight on that site, but no other information was available other than the takeoff/landing time.

Hmm, not sure where you can find more information then. Sorry :-\

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Aircraft emergency oxygen: how hard do I pull on the mask?
AKA 'why is the safety briefing/training inadequate'.  I tried Googling my title and got lots of sites explaining the emergency oxygen systems, procedures etc. that I already mostly knew.  The briefing: 'Pull sharply on the tube to start the flow of oxygen' is not exactly fully informative or reassuring.  The things look so fragile that it must be fairly easy to rip the tube away from the oxygen generator/supply bus pipe/whatever.
How am I supposed to know how hard to pull?  Is there some sort of click/detent that can be felt as I pull on the tube that indicates 'OK, you've turned the oxygen on'?
Could not a simple mechanical flag indicate oxygen on, eg 'Pull on the tube until the indicator flag turns green', or the like?

Perhaps a more informative question would be to ask what the mechanism that triggers oxygen release is. Knowing that should give you an idea what kind of pull would work best to avoid damaging the equipment.

This is a question that I also thought of, while in the airplane. But just think of it that way: The FAA will only consider the plane airworthy if you can't rip the mask out trying to follow the safety instructions. Their european counterparts even tested it with the average John Doe: They put some John Does into a mockup plane, play them the safety video, and watched whether/how they helped themselves once the oxygen masks fell down.

When they first fall down they'll be too high to reach your head, pulling on them will release a switch to activate the generator and release enough slack in the line to reach down to your chest comfortably (for children).
Some types are suspended by a cord connected to the switch:

(image source: wikipedia)
Then you will just need to pull until the cord disconnects from the ceiling.

Any idea how much oxygen is available for the passengers, for example, enough for 5minutes / 15minutes / 30 minutes / 6 minutes

@Firee 15 minutes

Thanks.. One more small query, can't they use re-breather's instead?

@Firee nope you need the oxygen at that altitude by the time you put on the rebreather you are already breathing the thin air, and after a while the air inside the rebreather will be further starved of oxygen.

You can't really pull too hard.  The tube (or a separate cord) is held in place by a pin.  As you pull down, you will pull the pin out and trigger the oxygen generator.
Once the pin comes out, there will be much more slack tube in your hand and the resistance will drop to zero.  At that point, you shouldn't need to pull any more.
Instead of thinking how hard to pull, just think that you need to pull the mask to your face.  If you do that, it'll work just fine.

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Is a safety pilot required to share costs?
Where I fly, it is customary for a pilot maintaining instrument currency to pay all the costs associated with the training flight while a safety pilot volunteers their time in exchange for the opportunity to log PIC. I personally don't disagree with this arrangement, since it encourages pilots to help each other out.
The FAA has repeatedly taken the position that building up flight time is considered compensatory in nature when the pilot does not pay the costs to operate the aircraft (see Harrington 1997, Lincoln 1990, and Howell 2013).
So, in this case, is the safety pilot being "compensated" by logging PIC? I vaguely remember seeing something saying that the FAA did NOT consider this compensation, but I can't remember where I saw that...

This actually came up in chat recently -- the answer (if only the two pilots are on board) is a qualified "No", not because the time isn't "compensation" but because the safety pilot is a required crewmember for simulated instrument flight.
Since both pilots are required crewmembers the safety pilot is not acting as a PIC of a flight carrying passengers as long as they are only acting as the Legal PIC while the other pilot is under the hood.
(Note however that while neither pilot is in simulated instrument conditions the safety pilot is not required crew, and therefore someone is PIC of a flight carrying at least one passenger - so log your time carefully and try to wrap your head around a "pro rata share" of that mess.)
This is further addressed (more directly) in the interpretation issued to Roberts, July 11, 2012.

Note that if we're talking about a flight with more than the two pilots on board (carrying someone who is NOT a required crewmember, like a friend in the back seat) then the whole thing goes out the window: No matter who is acting as the Legal PIC for the flight they're "PIC of a flight carrying passengers", and you're back to pro rata share territory.

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Carbon footprint & fuel efficiency vs flight distance
I was looking into my carbon footprint related to flying and seem to have came across somewhat of a contradiction.
WIKIPEDIA says "mid-range is better"
https://en.wikipedia.org/wiki/Fuel_economy_in_aircraft shows the fuel efficiency as a function of distance stating that the optimum distance is somewhere in the mid-range. Specifically

Above a certain distance it becomes more fuel-efficient to make a halfway stop to refuel, despite the energy losses in descent and climb. For example, a Boeing 777-300 reaches that point at 3,000 nautical miles (5,600 km). It is more fuel-efficient to make a non-stop flight at less than this distance and to make a stop when covering a greater total distance

IATA Carbon Footprint Calculator: "longer is better"
I used https://www.iata.org/en/services/statistics/intelligence/co2-connect/ to calculate the carbon emissions as a function of flight distance. In this case I use Newark as the starting point and choose different target cities averaging between both directions and using a Boeing 777 as aircraft wherever possible.
Despite their claim to be accurate and based on real airline data, the IATA numbers could easily be explained with a very simple model of
$$C = 59.2kg + D \cdot 0.056 \frac{kg}{km}$$
where $C$ is the weight of the emitted Carbon and $D$ is the distance flown. For most cities the error of this model was less than 1%. It's a simple linear model meaning "starting and landing produces 59kg and then it's a constant 56 kg per 1000 km distance flown".
Based on this model, a longer flight will always be more efficient (in terms of emissions per distance) since you amortize the emissions for starting and landing over a longer distance.
For example, I checked EWR->DXB vs EWR->FRA->DXB. The total distance is almost the same but emissions for EWR->FRA->DXB come out to be 739kg vs 669kg for EWR->DXB.
This seems to contradict the Wikipedia article which indicates that EWR->FRA->DXB should be more fuel efficient.
Questions:

Can someone reconcile the apparent contradiction?
If I want to minimize my carbon footprint should I split up ultra long hauls (where reasonably possible) or not ?

Breaking up long haul flights in shorter segments may reduce the carbon footprint, if the detour made is only small and the shorter flights are flown on aircraft that have comparable or better efficiency that the long haul flight.
Copied from this answer:

The uplift of fuel costs approximately 4% of its own weight per hour
of flight. That means the fuel you consume in the 4th hour of flight
has burned approximately 11.5% of its original weight, just to carry
itself. The fuel burned in the 7th hour of flight has burned 25% of
its original weight , just to be carried for the previous 6 hours.
(which means you have to load 133% of the fuel used in the 7th hour)
So a 10% detour in distance, with an extra stop, does not increase the
total fuel consumption much. It may even reduce the total fuel
consumption of the flight.

That answer looked at breaking up a transatlantic flight into two segments with Iceland as a halfway stop. The longer the flight, the more positive the effect of a stop in between can have.
It seems that the IATA calculator does not take into account that it costs fuel to carry fuel. They have made a linear approximation to a nonlinear curve.

In addition to the factors mentioned by @Ralph J, also realize that the airlines do not operate identical equipment on all routes.
In the US, there are three general categories.
The big-name airlines do not directly fly the shorter, regional flights to smaller cities.  These are handled by subcontracted regional carriers operating regional jets (often ERJ's or CRJ's).
On domestic routes between larger cities, the airlines typically operate their own, somewhat larger equipment.  Typically a B737 or A319/320/321.
On international routes, the airlines operate long-haul aircraft such as B777, B787, A330, A350.
The airlines have a direct financial incentive to minimize fuel burn -- fuel is a big contributor to their operating costs.  Consequently, they work hard to match the right aircraft to every route -- taking into consideration range and payload capacity (matched with demand for that city-pair).
Even within a gross model, there are lots of differences.  For example, there are four generations of 737's that have been produced (or are being produced).  The first entered service in 1968 -- airlines typically get about 25 years of service out of a 737.  Each generation typically has three models (small, medium, large) and sometimes they come in long-range variants, etc.
Each new generation comes with a new generation of engines that are substantially more fuel efficient than the one that came before.
Breaking a given trip into two shorter trips may switch you from a newer, more efficient aircraft with good load factor (many seats filled means better seat-mpg) to a less efficient aircraft with possibly less load factor.

The IATA formula, at least as summarized, ignores the problem that increasingly longer range requires increasingly higher takeoff weights, and more fuel used during the first part of the flight to carry fuel that's needed for the latter part of the flight.  At some point, the inefficiency of taking off extremely heavy and initially cruising at low altitudes (driven by the heavy weight) starts imposing its own penalties.  The first 1,000 miles (or km) flown by an aircraft near its max gross weight burns far more fuel than the first 1,000 miles flown by a lighter aircraft, all else (i.e. aircraft type) being equal.
If you assume that the "last 1,000 miles" flown by both the super-long-haul aircraft and the medium-haul aircraft are both flown at the same (comparatively light) weight and at the same (high, fuel-efficient) altitudes, then both would have comparable fuel burn per mile for that segment.  The problem is that the first 1,000 miles of the super-long-haul jet burns far more fuel per mile, and at some point those inefficiencies will overcome the benefit of "amortizing" the fixed fuel burn across a greater number of miles. (Also, that "fixed" burn for takeoff + initial climb isn't really fixed -- it also goes up as takeoff weight increases.  The descent & landing would be comparable, since they're both at basically whatever weight is assumed for landing.)
As far as

If I want to minimize my carbon footprint should I split up ultra long hauls (where reasonably possible) or not ?

It doesn't matter.  The airplanes are flying with you or without you, and the delta in burn between having +1 person on "this" flight vs "these" flights is too minuscule to contemplate.
Airlines may or may not actually care about CO2 emissions (they certainly want to look like they care, but in reality...), but they most certainly care about the cost of fuel, which is where the carbon comes from.  If there was a flight that was burning so much fuel that they'd make more money by flying it as 2 segments vs 1, or 1 instead of 2, then they'd do that.  The fact that both the nonstop EWR-DXB and the stopover in FRA exist, reflects market dynamics, the airlines' networks, & what's profitable, and neither is going to go away because those who work to compute their carbon footprint choose whichever one.

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During which part of the flight the effect of air resistance is more important?
Her teacher asked this question to my niece (8 years old)? However I couldn't find a solid answer either. In which part of the flight, the effect of the air resistance (drag) is more important? Takeoff, cruise, descent or landing?

I am not sure what the question even asks for. What of "effect of air resistance" (I am not sure it means the same as "drag", because induced drag is not caused by friction) should be "important"? It's existence? It being large? Or it being small? And what should it be important for? Keeping it the air at all? Minimizing the cost? It seems to be really ambiguous.

Perhaps you could explain what is meant by "more important". More important could mean most desirable, in which case the answer would be on landing. But if you are meaning most important because it has the most detrimental impact, then it might be cruise. If you can clarify this I'm sure someone can explain parasite and induced drag, etc.

Aerodynamics for 8 year olds? What sort of school is she going to?

@Jaydee A good one I suspect :-)

I agree with @JanHudec. Question is ambiguous. And now I see your answers which are all true in their own way. Maybe the question is asked in order to make the children think instead of getting a unique answer in response.

I think that this would be a very interesting question if given the appropriate context.  As it stands now, I believe that it is too broad to be given an accurate answer.  Please clarify before it is closed as such.

@Lnafziger As I said in the question, I did not ask the question originally. It is asked to my niece by her teacher (as a homework). If I try to clarify it, it will be just my version.

@NuriTasdemir I understand, but in the current form it is not answerable.  Perhaps ask your niece what they have discussed about this, or even clarify with her teacher so that an appropriate answer may be provided (both for her and on here).  :-)

The answer they are looking for is probably cruise. This is generally the longest part of the flight, so the losses from drag will add up. Most aircraft are optimized as much as possible to have good performance in cruise to save fuel and meet range requirements, so having low drag is more important in this part of the flight than others. This is also generally the section where the aircraft is traveling fastest. As Peter Kämpf showed, drag is highest at higher speeds, other than periods where landing gear, flaps, etc., are extended and add more drag.
As Jay Carr explained, drag is important at every point while the aircraft is in the air, affecting different areas of performance. During takeoff, drag will affect the runway distance needed, and initial climb performance. Low drag during descent is helpful, unless you need to get down faster, like in an emergency descent. Drag is probably highest during landing, because the emphasis is placed on having as much lift as possible at slower landing speeds, and this lift is created at the expense of higher drag.

I would guess that the aircraft's mass (because it takes time to accelerate mass) probably has more of an effect on the necessary runway length than air drag has; so I agree with you: cruise.

Also as Peter notes in his answer total drag is higher at higher flight speeds. Most aircraft cruise at a speed much higher than their takeoff or landing speed, so I concur: "Cruise" is probably what they're looking for.

@voretaq7: A well designed airliner will fly at the minimum drag point in cruise. Flying high is all about lowering dynamic pressure for a given speed.

@PeterKämpf True, but not all aircraft are modern airliners (If I fly my Cherokee at the minimum drag point I might die of boredom before it's time to land)

The question leaves a lot of room for interpretation, so here is my version:
If we assume a constant mass of the aircraft, resistance varies with speed, and there is a minimum relative to the lift force when the lift-induced and zero-lift drag components are equally large. You can now argue that air resistance (however defined) is least important at that speed and becomes more important at low and high speed.

The plot shows the drag components of a typical glider in straight flight, but if you adapt speeds and forces, the same plot works for any other airplane. It shows that the drag (= air resistance) is highest at the highest flight speed.

Anytime air is moving over the surfaces of the aircraft, drag is important.  There really isn't a time when it's "more" important.  When an aircraft wants to move, during any phase of flight, the thrust of the craft must exceed the drag the air (or wheels, on the ground) is creating on the craft.  At any point that the aircraft is flying, the drag must be overcome to the point that the wings can create the required lift to get the aircraft into the sky.
Perhaps, though, the question actually had to do with air brakes?  When those are deployed the amount of drag created is increased by quite a lot, slowing the aircraft by quite a bit.  And as a result air brakes are often used during the descent phase of flight in order to keep the plane from accelerating to much as it approaches the runway.  Perhaps that's what the teacher is after...

Drag (Air resistance) plays a part in all phases of flight, its most critical during takeoff or landing as this is when it will have the most drastic/immediate effect.
I would pick landing as my favourite for the answer and as it is the most apparent, with all the bits of metal hanging from the aircraft.
As the aircraft slows it lowers the leading edge slats(if fitted) and trailing edge flaps to increase the amount of lift the wing is able to produce, as you increase the lift you are also increasing the drag. In jet aircraft the spoilers are often used to assist this reduction in speed or to maintain/increase the rate of descent whilst decelerating(not easy in a heavy jet with lots of inertia). At some point during the approaching (normally around 2000ft ish) you have to lower the gear, which again adds to the drag.
All this drag on approach allows the engines to spool up, which is useful on large turbine aircraft as it means the engine is at a good state to provide power quickly in the event of a missed approach.
On touch down the configuration changes again - the spoilers deploy, reverse is selected and as the nose lowers the wheel brakes kick in - lots of drag (ok, the wheel brakes don't provide air resistance, but they are a fairly large part of the decel process). The spoilers drastically reduce the lift of the wing pushing the aircraft down onto runway meaning the brakes have the greatest effect. Some aircraft at this point change the flap configuration, in light aircraft its an option to retract the flap which reduces the lift on the wing making the brakes more effective. Other types lower more flap, having a "barn door" type effect. The Boeing 747 retracts the inboard section of slat, which again helps reduce the lift on the largest part of the wing.
As all this is happening thrust is redirected forward and power is increased on the engines helping the deceleration down to around 80-60kts.
In some aircraft (normally fighters) you hold the nose off the runway on landing to increase drag, braking aerodynamically down to a low speed. In commercial aircraft you normally lower the nose as the wheel brakes have greater effect (you normally have a lot more main wheels/brake units than fighter types).


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Garmin 530: What is NXRG telling me?
I have a Garmin 530 in a Cessna 550.
It has a constant display of NXRG:x:xx in the upper right corner of a map page. It changes color (yellow and green are common) and the value of x:xx changes to some sort of code (see pictures)
Any idea what this is telling me?

It looks like the "weather product age" for NEXRAD weather data, i.e. how old the current data you're seeing is (see Pilot's Guide, p.13). The number is contextual, based on the weather information type:

The age of the displayed weather product—or the effective time of
Winds Aloft predictions—is shown in the upper right corner of the
display. For example, if NEXRAD is displayed, “0:05” indicates that
the data are five minutes old. If Winds Aloft predictions are being
displayed, “10:00” indicates the effective time for the displayed
prediction is 10:00 AM.

In the first image, the data is 15 minutes old, in the second it's 6 minutes old.
The information and images on p.72 show that NXRG is "Region NEXRAD" and NXCO is "CONUS NEXRAD". P.70 also mentions NXRD, which seems to be just "NEXRAD" and it's used when no data is available.
As for the color, the manual says:

When the age of the displayed XM Weather product reaches one half of
its expiration time, the color of the displayed time changes from
green to amber

NEXRAD data is good for 30 minutes per the Guide, and that matches with the color change at 15 minutes in the first image.
Note: although the manual talks about SiriusXM providing NEXRAD data, ADS-B radar data is also from NEXRAD. Presumably the display uses the same abbreviations, regardless of which service provides the NEXRAD data.

I brushed over that because the airplane does not have XM data. It DOES have ADS-B In however and this may be subject to the same display. In the manual, it has that data line as NXRD and the accompanying photo has NXCO (p. 70 same document.)

@dawg That would make sense, ADS-B radar is NEXRAD too. I'll update my answer to clarify that; thanks for sharing that info!


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What is the theory of the ram's horn yoke?
Embraer tend to use a Ram's Horn yoke in the Phenom and many of their RJ aircraft:

And Cessna uses a more standard yoke with vertical sides:

What is Embraer's reasoning for the Ram's Horn Yoke? Are there advantages?

It was used on Concorde also.


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Why do airlines split themselves into domestic and international subsidiaries?
As an example, Air Canada operates its own brand for international flights, but has a subsidiary called Air Canada Rouge for domestic and US flights. What's the reason for creating such subsidiaries?

Air Canada does NOT split its route structure as you have suggested. Air Canada (mainline) does not ONLY fly international routes. They also fly  domestic and USA routes. Air Canada Rouge also does domestic, USA, and international routes. The routes often overlap but are generally different.

To be specific, Air Canada Rogue is intended to compete in the low cost carrier business, against WestJet, Sunwing and Air Transat. Similar splits include Eurowings (Lufthansa), Ted (United), Song (Delta), Scoot (Singapore). For real geographic splits look at the former Air France/Air Inter, BEA/BOAC, Cathay Pacific/Dragonair, KLM/KLM Asia, Continental/Continental Micronesia, EgyptAir/Air Sinai etc.

Sometimes it can be for the purpose of recruiting pilots and cabin crew at lower pay. The airline may have pay agreements, possibly union collective agreements or else just formal agreements with non-union "associations", which set pay, working hours, and other conditions. This can be a barrier to offering ultra-low-cost tickets on the discount brand. If they structure the low-cost brand as a nominally separate entity, those agreements won't extend to it. See, for example, this news story on Westjet's new low-cost brand.
But there are lots of other brand-management reasons they might do this. They may want the big brand associated with reliable, high-service business travel for which they can charge full fare; and the new brand associated with cheap fares at the expense of everything else. Look at how the car companies spawn different brands so they can focus their marketing on luxury (Infiniti) mass-market (Toyota) and fun-and-cheap (Scion, now rolled into Toyota).


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How often does evacuation happen as shown during the pre-flight safety videos/stewardess instructions?
I believe it is mandatory for airlines to provide passengers with information about evacuation in case of emergencies. However, pretty much every accident I've ever heard off either happens at the airport (where normal exit routes are available) or in the middle of nowhere (where no passenger makes it out alive).
Is there any statistics on how often do "normal" evacuations happen among Western airlines? The only example I can think off is the Hudson landing of US Airways Flight 1549.

You're right that the vast majority of evacuations happen at airports, but that doesn't mean they're using the 'normal exit routes'. I don't have any hard data for you, but head over to AvHerald and search for "evacuation via slides".

That's a great link! Thanks

I would imagine that the evacuation happens more often in the filming of evacuation scenes than it does in real life... but I have absolutely nothing to back that up with.

@mah: I don't think so. Precautionary evacuation is quite common.

I didn't do statistics, but The Aviation Herald lists units to low tens a year worldwide.
Most of them happen at airport due to fire indication (sometimes even false) or smoke on board.
On the ground, fire spreads a lot faster than while flying, because when flying, the wind is too strong and blowing the flames off. The most notable example would be China Airlines B738 at Okinawa on Aug. 20th 2007, airplane in flames arriving at stand. The aircraft was already at the gate when the flames were first noticed, evacuation was immediately initiated and by the time everybody left, it was already engulfed in flames, about 3 minutes after the fire started. While China Air might not be "western" in strict sense of the world, it is largest airline on Taiwan and follows the same standards.
Another notable accident is Egyptair B772 at Cairo on Jul 29th 2011, cockpit fire.
Here are some recent evacuations mentioned on AvHerald:

Air France B773 at Paris on Jul 28th 2013, evacuation while boarding due to fumes
US Airways B752 at Kona on Jun 8th 2013, evacuation during push back causes 4 injuries
PSA CRJ2 near Philadelphia on Apr 17th 2013, smoke on board, evacuation
Easyjet A319 at Belfast on Jan 6th 2011, taxiway de-icing prompts evacuation

Notice, that they are all smoke or fire events. For some other reasons, see e.g.:

Wizz A320 at Rome on Jun 8th 2013, left main gear did not extend
Alitalia A320 at Rome on Sep 29th 2013, right main gear did not extend

two similar accidents where both aircraft were evacuated via slides. The reason in these, and many other gear collapse incidents, is that they don't want to wait in a damaged plane where there is a risk of fire, plus the difficulty of getting stairs to the aircraft resting in strange attitude.


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What stops an international private airplane from landing outside of airports with a customs zone?
Let's say somebody charters a small airplane from Canada and flies into the US (after receiving clearance from the US ATC). After crossing the border, they land on a small airstrip in the middle of nowhere instead of going to an airport with a customs facility.
Would anything stop them from doing so? Would the police be notified immediately by the ATC? Is all international air traffic closely monitored until they pass through customs?

If one wanted to smuggle across what is famously "the longest undefended land border in the world", there would be easier ways than using a plane!

@AakashM a simple hike would do

For the purpose of this question we can assume it's an otherwise harmless US citizen who's carrying zero taxable goods.

What happens is it crashes at a large airport and nobody notices for hours.

In theory, if one would want to land at an unplanned airport, one could feign mechanical or medical issues. However, landing at an airstrip in the middle of nowhere for this is kinda weird.

@NateKerkhofs for mechanical issues when you need to get tot he ground NOW and the middle of nowhere strip is closest

http://en.wikipedia.org/wiki/Mathias_Rust even the USSR failed to act on an intrusion like that, and their air force had standing orders to force to land any intruder and shoot if that didn't work... The US are more friendly.

Just admit it: you were not thinking about the Canadian/US border, but in the Mexico/US border.

@fooot Did you hear more details about that guy? Media dropped that story altogether just after a few days.

Out of curiosity, why the constraint of getting clearance from ATC? You don't need any clearance from ATC to fly VFR. You don't even need to file a flight plan. As far as radar coverage is concerned, given the very remote and occasionally mountainous nature of much of the U.S.-Canada border, I'd be willing to bet that there are parts of the border where it would be possible to stay below radar coverage long enough after crossing that it wouldn't be obvious that you crossed the border rather than having taken off from one of the numerous uncontrolled fields lying around in the area.

It is usually required to file a flight plan for international flights (with very few exceptions), and I'll assume this is the case for Canada/US as well, but I'm not 100% sure of this. In this case you have two options

You close your flight plan and let ATC or FSS know where you are, they'll note it's an international flight that landed somewhere without customs, and you're likely to end up in trouble.
You don't close your flight plan, and you'll trigger a search-and-rescue operation as you'll eventually be overdue, drawing attention to yourself, and you're equally likely to end up in trouble. Not to mention that you'll probably be liable for the cost of the search-and-rescue.

You could of course file a plan with a drastically reduced TAS, do a very quick stop, and arrive at the original destination within the grace period. I don't know if that will trigger any surveillance systems though.
If you don't file a flight plan and cross the border anyway, I'm guessing that you will indeed trigger some alerts though.

So, to answer, the OP, it's legally required that you go through customs the first time you land in the US, and if you chose to go to a field with no customs office (whether in a filed plan or not), you would be in trouble? That is, assuming you don't have a good excuse such as an engine failure. In such cases, are you supposed to notify the local police to get on record as having "properly" entered the country? I've heard of international flights being diverted to an airport with no customs (or closed for the night) and they kept everyone on board all night.

@PhilPerry In the event of an emergency landing, all persons and cargo must remain on board the plane until customs and immigration agents arrive to admit them legally—whether that takes one hour or ten hours. If the plane is on fire, they might make an exception.

Nothing would stop them from doing this unless someone dropped a dime on this flight with credible evidence that this would happen.
Yes police or the DEA would be notified.  This is the profile of a drug smuggler.
If you act like a drug smuggler, expect to be treated like a drug smuggler.  Is all traffic scrutinized?  I cannot confirm that all traffic is so closely monitored.  But much of it is.  Currently there has been more than a few encounters with local police notified by the CBP for "suspicious" flight between California and points east with no border crossing and no probable cause.  These flights have been met with up to a dozen officers with arms drawn.  Illegal stops, IMHO but I am no lawyer.  So if they stop a plane after it flew from San Diego to Albuquerque (as one example) it is highly likely that they would meet a plane that diverted to a different destination than it had filed for, coming in from Mexico or Canada.

When considering flying an aircraft across the U.S. border, you also have to consider that there is extensive radar coverage of all airspace within the U.S., with varying flight ceilings for different classes of aircraft. Monitored not just by the FAA, but also the Department of Homeland Security and the Department of Defense.
This isn't just a safety issue of resolving conflicting flight paths, maintaining air corridors, managing airport capacities, and avoiding collisions. This is also an issue of safeguarding national boundaries and preventing smuggling and terrorism by tracking authorized and unauthorized entries.
Entering the country by crossing into its airspace without filing a valid flight plan and having a valid visa/proof of citizenship would constitute a crime of illegal entry. Landing in an out of the way location without a valid customs zone could be construed as trying to avoid registering your presence to the U.S. government, declaring imported goods, paying taxes, or a more nefarious purpose.
The worst case scenario, depending on what the current DHS security level is, could result in you picking up an escort of F-16s to take you to a secured airfield where federal agents will take you into custody, do an extensive full body cavity search, grill you to the nth degree as to your origins and purpose, and detain you for prosecution on any number of charges that are based on what they dig up on you. And don't forget that the NSA has been collecting a vast amount of information on everybody.
Department of Homeland Security U.S. Customs and Border Protection: Pleasure boats and private flyers


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What are these vehicles with no callsign used for?
I keep seeing these land based vehicles whenever I am looking airports on flightradar24, like in the image below:

Why do they come up on the radar? Do they have a transponder? What is the purpose of these vehicles?

Tugs, fuel trucks, and those things. Possibly that the airport wants to keep track of where they are, so they have ADS-B transponders.

They have Mode-S transponders / ADS-B transmitters specifically for vehicles such a the ERA SQUID, Thales MOSQUITO or Saab Sensis VeeLoPDF.
Typically vehicles that operate on taxiways and runways are equipped with such transponders, which ensures the vehicles can be tracked by the Advanced Surface Movement Guidance and Control System (A-SMGCS).
Vehicles that only operate near parked aircraft are usually not equipped.
When using a magnetic mount, these transponders can be easily transferred between cars. This reduces the total number of transponders needed, but makes it necessary to reprogram the call sign after each transfer. For this reason it may have been decided to have no call sign in the transponder at all for these vehicles at Sydney airport.
An example of an airport using fixed installations with call signs is Amsterdam airport. Using FR24 you can see bird control, maintenance, supervisors, tugs, fire trucks etc. driving at the airport and sometimes,  when they forget to turn the system off, on the highway.

After some further investigation, I am fairly sure they use the Saab Sensis VeeLo in Sydney. The units are magnetically mounted to the vehicles. Each unit transmits using a fixed unique 24-bit address (like all Mode-S transponders /  ADS-B tranmitters) and no call sign. In the A-SMGCS software the 24-bit address is mapped to a call sign which is displayed on the Controller Working Position. There is procedure in place to ensure that when a vehicle is entering the manoeuvring area the correct mapping between the 24 bit address and the call sign is in the A-SMGCS system.

On this question it is mentioned that ground support vehicles are equipped with ground transponders that work with the airport ground radar. I doubt they have ADS-B. They are most likely getting a feed from the ground radar system. In this thread on the Flightradar24 forum they discuss what types of ground systems are compatible with the FR24 recievers. One of them mentions that the feed can apparently be switched off by the controllers.
You can toggle ground vehicles on and off in settings,


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What happens when disengaging the electricity generators on a B747?
This answer says:

If let's say, you [...] disengage the electricity generator of every engine (which cannot be re-engaged without mechanical maintenance), [...].

What exactly happens in the aircraft when disengaging the generators and what benefit does it have to be not able to engage them again?
Is it only the B747 that works like that or also other Airliners?

Speculation, but to re-engage them in flight would require a clutch mechanism (with added complexity and weight).  For something that is rarely used, a disconnect-only mechanism that can be rejoined when the engine is stopped is probably cheaper and lighter.

Re-engaging a disconnected drive, if you could do it, would risk again incurring the problem for which it was disconnected. Since you've got three other generators, better to not take that risk I would think.

@Terry In general I'd agree, but do note that OP's quote says every engine. So you've got zero generators running.

@MichaelKjörling Ah, another senior moment on my part. As they say, RTFQ.

You have 3 questions:

What exactly happens in the aircraft when disengaging the generators?
What benefit does it have to be not able to engage them again?
Is it only the B747 that works like that or also other Airliners?

What exactly happens in the aircraft when disengaging the generators?
As mentioned in the other answers:

A solenoid removes a retention pin.
This allows a spring-driven pawl to engage a worm gear on the drive input shaft.
This causes the inner input shaft to disengage from the engine drive.
Thus the generator drive rapidly coasts to a stop.

Source: k-makris.gr/AircraftComponents/CSD/C.S.D.htm

What benefit does it have to be not able to engage them again?
The primary reason to disconnect the generators is when some mechanical problem causes a risk of serious mechanical damage to them.  From the Boeing 747-400 Abnormal Procedure ChecklistsWord Doc :

Condition:    Low IDG oil pressure or high IDG oil temperature.
Crew Response:
          Affected GENERATOR DRIVE DISCONNECT switch PUSH.
          Note: This action prevents damage to the IDG.
          DRIVE DISC message is displayed.
          ELECT GEN OFF message is displayed.

A lesser reason might be to prevent a seized-up generator drive from adversely affecting an otherwise operable engine.
Since whatever caused the loss of oil (pressure) and/or overheating almost never can be corrected in the air, then there is no point in being able to reengage the generator drives in the air.
Furthermore, to provide that capability would require complex mesh gears, clutches, and mechanisms that would add considerable weight and more points of failure -- all to cover a contingency that is both rare and very unlikely to be critical (discounting sabotage).

Is it only the B747 that works like that or also other Airliners?
Most large jet aircraft use some form of Constant speed drive (CSD) to power their electrical generators, as the alternatives are not well suited to the large electrical demands of jetliners.
From Wikipedia, we see that newer Boeing and Airbus models use a single-case variant of this called an "IDG".

This is how it works on all aircraft with a Constant Speed Drive (CSD) Generator or an Integrated Drive Generator (IDG) - doesn't matter who the airframe maker is.
When you press the disconnect button (the red arrow), the disconnect circuit is completed and the solenoid pulls the pin out of the pinion shaft (the blue arrow) which then engages the worm gear on the drive shaft (the green arrow) and causes that gear to disengage from the drive mechanism. This causes the generator to stop turning.
To reset the mechanism you have to pull the "RESET HANDLE". This drive mechanism is attached to the gearbox on the engine which means that it can only be done on the ground with the engine off

If you want a slightly better image (at the end of this page).

For some reason I find this to be an amusing method of disconnecting a gear.

The button I was talking about is the "Drive Disc" button, circled in red here:

source: http://www.meriweather.com/flightdeck/747/over/elect.gif
This button mechanically disconnects the Integrated Drive Generator (IDG), a generator for providing electricity, from the engine. The button above it disconnects the IDG electrically from the buses, but keep the IDG connected to the engine (so it is spinning as the engine spins).
The disconnect button is pushed when there is reason to believe that continue operation of the generator may lead to engine damage, for example when there is a low oil pressure or high temperature indication of the IDG.
A similar configuration exists on Airbus as well, see for example this YouTube video where disconnecting the drive is part of a procedure for abnormal engine indications. As a result of disconnecting the drive, the electricity output of the plane is reduced.

Thanks! What can I imagine when you say "mechanically disconnects"? And why is it not reversible?

@NoahKrasser I cannot find a description to the disengage mechanism in either the Flight Crew Operations Menu, Flight Crew Training Manual & the Quick Reference Handbook, so the answer do that is probably in some maintenance handbook (which I do not have).

Ok, thanks for your effort!

But this didn't answer the "what happens " part of the question, but only the 747 vs Airbus part

@NoahKrasser "mechanically disconnects" often means "uses the sort of technology that an 18th-century engineer like James Watt might have invented". The final backup in a chain of protection devices needs to be very simple and very reliable. As Sports Racer's picture shows, you can't get much simpler and more reliable than "you yank a pin out of the drive gearbox, so the gear train irreversibly falls apart."


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What exactly happened with the Flight Computers on XL Airways Germany Flight 888T?
I recently saw a documentation about XL Airways Germany Flight 888T. This made some questions rising in my mind:
Two AOA-Sensors failed (freezed) simultaneously. From Wikipedia:

Two out of three angle of attack sensors have been frozen and rendered inoperative. System logic has been designed to reject sensor values which deviate significantly from the others. In this specific case, this principle led to the rejection of the single operative angle of attack sensor, and to the acceptance of the two faulty ones, which provided similar values, but have been stuck since cruise flight.

The Airbus "shut down" the computers because of unlogic values and displayed USE MAN PITCH TRIM. But as stated above, the Airbus rejected the third sensor and accepted the two working sensors.

What made the computers to deactivate and switch to manual mode?

Is it comparing the outside AOA-Sensors with the gyro data (Thanks to the user "mins" for clarifying the diffeence between AOA-Sensor and Artificial Horizon Data, but maybe the Airbus uses the Gyro-Data as a less ranked AOA reference, which flows into the detection of a faulty AOA-Sensor)

Why aren't Airbus-Computers switching to something like alternate law automatically, instead of deactivating the FBW and displaying a small warning?

It was really hard to form my questions and I'm sure I forgot something. But I would appreciate it if you could answer the questions.

We need to read the BEA report to understand why. It is not clear to me that flight computers were not available. The crew was actually performing various tests part of a scenario to demonstrate the protections in place, on a voluntary basis. More information is required.

Quite simply, if two of the three say 10 degrees and one says 5, the computers have no way of knowing which is the correct one.  The logic says if 2 agree and 1 doesn't, the 2 in agreement win.

“USE MAN PITCH TRIM” would indicate direct law (even less than alternate mode) and they would know it is pointless to test Alpha-floor in direct mode, so I don't think it was indicating that.

@Simon That's it, right. But the computer shut down a few seconds after it has accepted those 2 wrong sensors. Why?

@NoahKrasser, no, the computers never shut down. They did switch to direct law after the plane left the flight envelope. And no, I don't know what the analyst smoked when they decided it should switch to direct (where trim remains full up) rather than alternate (where the trim would move back on nose-down control input) law.

Of course they didn't stopped working, but they switched to, as USE MAN PITCH TRIM refers to, Direct Law. The flight computer should have been remained in Normal Law because there was no logical failure. What I want to know: What made the flight computer switch to Direct law? This occurs when there is a logical failure. What was this logical failure?

Tl;dr it was in direct law because air data was inconsistent and the landing gear was down.
For a full answer to why the aircraft did what it did I'll answer a few questions one at a time.
Why did the flight computers reject the good sensor and use the frozen ones?
The values of the sensors are fed to the control computers by the ADIRU (air data and inertial reference unit). There are three ADIRU's, each corresponding to three redundant systems of sensors. Part of the ADIRU is the ADR (air data reference). The ADR is responsible for determining the validity of the values coming from the air data sensors (pitot tube, static port and AoA vanes), correcting those values from local AoA to airplane AoA and feeding the values to the control computers. (local AoA at the sensor location is not necessarily the same as the overall airplane AoA due to their positioning on the plane.) Each ADR uses two resolvers for each sensor and compares these values for consistency. Along with the value it also sends to the control computers an indication as to whether the values are valid or not.
The ELAC (elevator/aileron computer), which controls movement of the flight surfaces takes the values from each ADIRU and compares them against the median value. If a sensor deviates from the median value past a certain threshold it assumes sensor failure and rejects the input. It then uses the average value of the other two.
Unfortunately for the crew of XL888t this method anticipates a single sensor failure. When two sensors fail at the same or similar value the system will reject the working sensor. There is really no way to overcome this, but having two sensors fail at the same value is exceedingly unlikely.
Why did the control laws degrade?
This is really the crux of the question. The ELAC is what determines the control laws. It uses information from the aircraft configuration (flaps, slats, air brakes, undercarriage)and the output of the ADIRU to determine how to interpret the pilot's control inputs. It uses this information to determine the α protection speeds (α-prot, α-floor and VLS) and when to engage the automatic envelope protections.
Normally when the aircraft slows the AoA increases unless a nose down input is given. In the case of XL888t the pilots were intentionally trying to put the plane into a stall in order to demonstrate the α protections. The elevator and stabilizer were in full up position and the engines were slowed. The ELAC will allow this position until it reaches the calculated values for α protection. In this case the AoA was not changing. When the parameters the ELAC is using get so far out of their thresholds the ELAC can no longer make the necessary calculations, so the α protections are disabled and control law is degraded to alternate.
So why did it go into direct law?
The test the crew was performing at the time was low  speed  check  in  landing  configuration." Landing configuration obviously indicates that the landing gear be down. In alternate law roll control is in direct law, but pitch control is still as it is in normal law, with automatic trim, etc.,except without α  protections. But when the landing gear is down the pitch control shifts to direct law and autotrim is disengaged. The "USE MAN PITCH TRIM" warning is displayed on the PFD.  It's the pilots' failure to notice this warning that resulted in the crash.
As to why the control laws are designed this way I can't say. Maybe someone else can explain why Airbus made that choice.
Note: All of this information was taken from the BAE final report.

Thank you! Answered my question perfectly and I could learn a lot new things about the Airbus Systems.

“There is really no way to overcome this”—no, but I would expect it to at least tell the pilots that it happened. For airspeed it does and there is an unreliable airspeed procedure to be followed when it happens. However for the AoA vanes it does not.

@JanHudec I guess Airbus thinks the way the BEA does. From the BEA report: "Angle  of  attack,  though  significant  for  the  study  of  the  aerodynamic  situation of  the  aeroplane,  is  not  a  piloting  parameter."


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What's that "frame" in the windshield of some piper aircraft for?
I often asked myself what that "frame" on/in the windshield of some Piper Aircraft is. Below is a photo:

What I thought it could be, is a "runway alignment help" for the pilot. But I have no idea.
Thanks for all answers.

What model of Piper aircraft do you see them in?

@SMSvonderTann They are not in all models. I recognized them in a Seneca and in a PA-31 Navajo.

As Thunderstrike suggested, these are windscreen heaters commonly marketed as "hot-plates".
The Piper Aztec I flew had one of these. When I first started flying the aircraft I expected some difficulty in seeing through or around it, as it was directly in my view. However, when actually flying the aircraft, I hardly noticed it was there.
As with all the electric windshield heat that I am familiar with, it is intended to be used as an anti-icing measure, not a de-icing measure. This means that the plate is heated so that frozen water that strikes the plate should melt and flow off, and liquid water should not freeze but should flow off. The heat is provided by fine, high resistance wires running vertically between two plates of laminated glass. The wires are visible if one focuses on them, but as with looking through a window screen, the wires disappear when one focuses on the distance.
See the following close up of a heated windshield employing the same type of heating elements:

Source: own work
These hot plates are available for multiple makes and models of aircraft, both as a replacement heater for aircraft with inoperative windshield heat such as the Piper Navajo, and for aircraft without installed windshield heat such as the Piper Aztec.
For this classification of aircraft, in order to meet flight into known icing conditions (FIKI), windshield heat is needed to provide an adequate view for taxi, takeoff, approach, landing, and certain maneuvers. Having an adequate view is helpful at any stage of a flight. However, for the practical purpose of anti-icing, windshield heat is arguably most important—and required—during the landing phase. See 14 CFR 23.775(f):

(f) Unless operation in known or forecast icing conditions is prohibited by operating limitations, a means must be provided to prevent or to clear accumulations of ice from the windshield so that the pilot has adequate view for taxi, takeoff, approach, landing, and to perform any maneuvers within the operating limitations of the airplane.

@Jules See my comments here regarding your question about the requirements for the anti-icing capabilities.

for those with no access to aircraft to look at the real thing, it's also an option on some cars, including several models of Fords.

Looks like it might be a Windshield "Hot Plate" for anti-icing. Electrically powered, heating from resistors warming the panel prevent freezing in that windscreen area.

Source

Source
They are available online from many vendors online if you search for them. The design is a lower-end solution to the heated windscreen:

Piper PA-31 Navajo Windshield Anti-Ice Hot-Plate Kit
This kit is desirable for the Piper PA-31 Navajo operator who doesn't
  want the expense and resultant down-time required to replace a heated
  windshield. Source

Can you provide a short explanation on how it works?

Is de-icing this small an area generally beneficial for longer flights, or is it "just good enough" for a landing at the nearest airport if the ice buildup is starting to seem hazardous? (Asking as a non-pilot)


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How did shortening the 707 increase its range?
According to the Wikipedia entry for the 707-120:

The 707-138 was a −120 with a fuselage 10 ft (3.0 m) shorter than the others, with 5 ft (1.5 m) (three frames) removed ahead and behind the wing, giving increased range. Maximum takeoff weight was the same 247,000 lb (112,000 kg) as the standard version

With the same MTOM between the -120 and the -138, what made the difference in the range? Was it simply less parasitic drag from the shorter fuselage?
I would presume that the fuel capacity (stored in the wings - center section tanks didn't come about until the -320) would have been the same.
I see notes on the -120B which switched to the P&W JT3D-1 engines which were more fuel efficient. That would account for greater range, but that doesn't apply to the -138 (at least not at launch).

Could it be as simple as less fuselage mass means you can fill up the tanks more if the (unchanged) MTOM rather than tank volume is the limiting factor?

MTOW is Empty Weight + Payload + Fuel Weight, and is limited by aircraft geometry factors. For a shortened aircraft, empty weight will be lower, which means either more payload or more fuel weight allowed. Some aircraft are payload limited at their extreme range.
Range is a function of MTOW and empty weight, according to the Breguet Equation for a jet:
$$R=\frac{V}{c_T} \cdot \frac{L}{D} \cdot ln\frac{W_i}{W_i-W_f}$$
with ${c_T}$ = specific fuel consumption, $W_i$ = initial weight, $W_f$ = fuel weight.
For a given fuel fraction and engine type, the primary operational parameter in the equation is L/D: the drag polar. Shortening the fuselage will indeed lower the $C_{D0}$ as OP states. Torenbeek section 5.3.1 gives some statistical data for high subsonic jets of the B707 era:

Typical $C_{D0}$ = 0.014 - 0.020
$C_D$ (including induced) = 0.02 - 0.12

At the same speed, engine type, MTOW (and L), fuel weight: a reduction of D of 10% results in an increase in range of about 10%.

A random drive-by upvote of the question got me to notice that I'd never accepted an answer...

If the fuse was shortened that much (reducing both the weight of the airframe and its cargo-carrying capacity) and the maximum takeoff weight stayed the same, the most natural explanation is that the plane's allowable fuel load at takeoff or the tank's capacity was increased.

Notice the robustly overdesigned wings and tail of the 707 readily allowed shortening of the fuselage.  An interesting study would be shortening vs thinning (reduction of cross section) of fuselage.

Also a shortened fuselage has a higher thickness ratio so in theory it could be made of lighter components while supporting the same loads. In practice this is not done for production economics, as it would be more practical to up rate the strength rating rather than what amounts to creation of an entirely new plane. Shortening also allows higher rotation angles for takeoff and landing so it could be a factor in runway length limits.(as it is with 737-900 vs -700)


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Was the Hughes H-4 Hercules capable of flying out of ground effect?
According to the wikipedia article the Hughes H-4 Hercules (Spruce Goose) reached a maximum altitude of 70 feet on its flight.  At this altitude the aircraft was still in ground effect.  Would the H-4 have been capable of flying out of ground effect?  Why or why not?

The answer can only be given by comparing it to other, broadly similar aircraft. My three references would be the British Saunders-Roe Princess, the Martin Mars and the Boeing 314 Clipper, all three at their time huge flying boats.

Obviously, the H-4 has the highest wing area relative to its installed power and its intended maximum take-off mass (MTOW), but does not look totally out of the range of the two smaller specimen. The Princess was conceived as a pressurized airliner and is of a more advanced design than the other three. With the lower wing loading come lower speeds, so the lower power loading looks adequate. Another huge military transport plane of that time which was derived from a cargo glider (the initial requirements asked for a cargo volume equivalent to a Reichsbahn standard cargo wagon) had a wing loading of just 150 kg/m², a very similar power loading of 23.56 HP/m² and flew just fine. Stick forces were massive, however, and stability was marginal.
The exact performance would depend on details like airfoils, aspect ratio, flap settings, hull design and stability, but in general it looks as if the H-4 would have been capable of flying out of ground effect. But its performance would not have been spectacular - a little more installed power would had made it better.


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What is the maximum Mach number for a titanium aircraft?
What is the maximum achievable Mach number for a titanium aircraft assuming speed is limited by structural heating?  The answer would be even better if there is a list of different materials and maximum Mach numbers.

The answer would be even better if there is a list questions asking for lists are not well received on SE.

The maximum achievable speed is usually due to aerodynamic (see coffin corner). I think your question is more about how to determine the surface temperature of an aircraft in relation to its speed at a given altitude (which is a good question) and what is the maximum temperature achivable without lost of strucutral integrity of a specific material (titanium) (another good question that should go on physics.SE)

I'm voting to close this question as off-topic because it belongs to http://physics.stackexchange.com/

Manu H The speed limiting factor of the Concorde was skin temperature, not aerodynamics.

@TomMcW I may not have been expressing myself correctly (perhaps because my mind was not clear enough). Given a targeted mach number (a bit more than mach2 during several hours for the Concorde), you choose both aerodynamic and skin material to achive our goal. No need to take a more expensive material if a cheaper and less heavy do the job. Thus, the question is more about determining the skin temperature depending of the speed.

@Manu H I think I said the wrong aircraft. I think it was the SR-71 and not Concorde. Peter Kämpf mentioned it in one of his answers but I can't find it now. They had to do some very creative things to that plane just to keep the skin from melting. Although it didn't use thrust to get it there, aerodynamically the space shuttle moved fast enough that skin heat was the limiting factor. And you see how tricky that was. Not sure if there are power plants that could overcome that kind of drag but they're working on scramjet technology that might get speeds beyond anything titanium can handle.

And btw, there's a company that claims to be selling jewelry made from actual skin from an SR-71. Not sure how they managed to get ahold of it

To give an answer, I first need to get answers to a few more questions:
First question: Which titanium alloy would you consider? The choice of alloy will impact the answer much more than the base metal of the selected alloy. Since strength at elevated temperature is the figure of merit here, consider the spread of values:

Strength of various Ti alloys over temperature (picture source)
Second question: Which cooling is available? Is the aircraft propelled by liquid hydrogen which has a massive heat capacity per kilogram of mass and needs to be heated before it is injected into the combustion chamber. Or do you run it on kerosene, which allows to dump much less heat into it? The answer will tell you how much cooler the structure can be than the air flowing around it is.
Third question: What wing loading is possible? This is driven by the range and payload requirements of the aircraft, and your question doesn't give any details on this. A higher wing loading will translate into a higher heat flow since the air density needs to be higher, so the cooling can be less effective.
Fourth question: Do you insist on an attached shock which produces much higher peak temperatures in parts of the structure, but less thermal load overall, or is a separated shock possible which increases drag and thermal load, but reduces peak loads considerably?
Depending on the answers to those questions, the possible flight Mach number can be anywhere between Mach 2 and Mach 5. If you allow for specialty materials for highest thermal loads and make only the inner structure from titanium, the Mach number can reach into double digits.


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What are the advantages of a Pelikan tail compared to a V-tail?
The Pelikan tail was originally proposed for the X-32.  Why was the Pelikan tail proposed rather than a V-tail?  What advantages and disadvantages does the Pelikan tail have compared to a V-tail or a conventional tail?

A Pelican tail is a V-tail with the two tail surfaces set apart. Compare this to a straight V-tail as that of the Lockheed F-117:

Tail view of the F-117 (picture source)
The closest to a Pelican tail which has actually been flown is maybe that of the Northrop YF-23:

Tail view of the YF-23 (picture source)
Clearly, the distance between the two surfaces is dictated by the engines, but Northrop could have placed them on top of the nacelles. However, doing so would had been a poor choice for several reasons. A Pelican tail has those advantages:

If the vertical part is too close to the wing strake or fuselage chines, the strake vortex could hit the tail surfaces and cause oscillatory loads. The F-18 vertical tails needed to be reinforced so they would not break off. A Pelican tail should easily avoid this.
A pure V-tail has poor pitch damping, because the pitch-induced angle of attack change is reduced and the reaction force, too, by the V angle. Placing a horizontal strip in the middle improves pitch damping.
Wikipedia has conflicting information on weight savings, but generally V- and Pelican tails should be more efficient than a conventional tail. However, for the best maneuverability a conventional tail is preferable. Again, placing a horizontal part between the two V surfaces improves pitch maneuverability, even when the control surfaces are only on the V part. Their symmetric deflection will induce a considerable lift change on the horizontal part.
Placing the vertical tail outside and aft of the engine exhaust will shield the lateral IR radiation better.

The drawbacks are:

Higher structural complexity compared to a true V-tail.
Lower maneuverability and yaw damping than a conventional tail.

EDIT:
I knew I had seen a Pelican tail before, but could not remember where. Now I found it:

This ist the German Fletter 282 V23 helicopter (picture source), and yes, this is a Pelican tail; however without control surfaces. It had two intermeshing rotors which made a tail rotor unnecessary. Anton Flettner had built the similar Flettner 265 in 1940 and introduced the 282 in 1942.

Was waiting for this.  Thank you for the great answer to my question.

The X-32 was Boeing's entry for the JSF contest, which was supposed to serve USAF, USMC and USN. JSF called for an affordable VTOL capable aircraft operated by all three services. Boeing decided to go for a tailless single piece delta with a direct lift system, similar to the Harrier.

Image from militaryfactory.com
The adoption of direct lift system and other considerations meant that X-32 had weight issues (They actually had to strip off some parts for VTOL). However, midway through the program, USN changed its requirements, requiring improved performance and payload capabilities. This caused serious problems for Boeing as its present design was expected to fall short of the requirements.
In order to overcome this, Boeing decided to change the wing and tail design of X-32. For tail design, two designs were proposed- a conventional four poster tail (like Lockheed's X-35) and a Pelikan tail (it was conceived by Ralph Pelikan, who worked for McDonnell Douglas and now for Boeing after their merger). The main points put forward in favor of Pelikan tail were,

It caused less drag compared to conventional tail
Offer pitch control at high AoA.
It had better stealth characteristics (two tails would be better than four).
The design would look different from the Lockheed concept.

However, the problem was that nobody has flown with this design so far. After some discussion, Boeing decided to go with the Pelikan tail. However, after a few days, the decision was reversed as the design was found to be heavier by a considerable amount as the larger control surfaces required beefier hydraulics and Boeing decided to go with a conventional tail, which turned out to be around 1500 lb lighter compared to the original design.
However, the prototype was not actually modified due to lack of time. The modified aircraft would've looked like this:

Image from aerospaceweb.org

Saab is testing a (sub-scale) concept with Pelikan tail called the FS2020, which has Pelikan tail.

Image from gripen4canada.blogspot.in


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Why do lower molar mass rocket exhaust products give higher specific impulse?
I tried to work this out.  My thinking is as follows.  If the same amount of energy is given to a low mass particle and a high mass particle, the low mass particle is faster.  A particle with a mass of 1 will be twice as fast as a particle with a mass of 4 but the momentum of the particle with a mass of 1 will be twice that of the particle with mass 2, which seems to go against what is seen with regards to molar mass and specific impulse.

I think this question might do better on the Space Exploration SE site.

Since specific impulse is per unit of mass. So you should not be comparing a low mass particle with a high mass particle, but a bunch of low mass particles with a smaller bunch of higher mass particles of the same total mass. And the molecular mass falls out of equation and only the total mass matters. Besides products of burning high molecular mass fuel (like kerosene) don't have high molecular mass themselves.

Your question is slightly confusing. What you seem to ask is why, given that momentum is proportional to the energy, and speed increases only with the square of the energy input, a heavy rocket fuel is so much worse than a light fuel.
For a rocket, the speed of the burnt gases exiting the nozzle is important. Heavy molecules need more energy for the same acceleration, so the lighter the rocket fuel is, the higher are the nozzle speeds. Momentum is less important, because the mass of the fuel is on both sides of the equation: Heavy fuel in the rocket's tanks needs more energy for the same acceleration, and the chemical energy of the fuel is not proportional to its molecular mass.
Thrust is mass throughput times exit speed. For a given mass, thrust can be increased if the exit speed can be maximized. Since this mass must be accelerated, the total fuel mass needs to be as small as possible, not the number of fuel molecules (as your question implies). As you say, using lighter molecules helps to achieve this. The only problem is that lighter fuels need more volume, so a hydrogen rocket is bigger than one using alcohol (V-2) or Kerosene (Vostok).
To get the best specific impulse, the mixture in a hydrogen rocket engine is not stoichiometric, but uses only as much oxygen as required to heat up the gases up to their dissociation temperature. Adding more oxygen would set free more energy, but this would only be used up to ionize the combustion gases, not to heat them up more (Oberth effect).


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Why is magnesium not used more often as a structural metal in aircraft?
Magnesium is lighter than aluminum for the same strength.  If the answer is corrosion, how much work has been done to improve the corrosion resistance of magnesium?  How much lighter would a magnesium structure be than an equivalent aluminum structure?

How would a post-crash fire interact with a magnesium skin?

Poor corrosion resistance is the major concern but it is used with aluminum to increase it's corrosion resistance.Recent studies found out yttrium also used for improving the resistance.

you obviously have never done the high-school chemistry experiment of setting a tiny piece of magnesium alight

You might want to look into aluminium magnesium alloys which have been used in aircraft.
You perhaps (?) seem to have the idea that it's a very much one-or-the-other deal: Note that even aircraft aluminium contain a mix of around 10% other metals for alloying. There's a huge variety of aluminium alloys and correspondingly their properties.

What I'm interested in is comparing alloys comprised primarily of magnesium with other materials.

Because it is expensive, a pain in the ass to machine and it burns like a son of a bitch.

@TylerDurden it will only burn in the liquid phase, so for e.g. for the aircraft interior flammability is not a problem per se as there is no hope of survival at melting point temperatures anyway. Corrosion is a big issue, though. And yes, passengers scraping of kindling and fiddling around with that also does not instill confidence.

@yankeekilo I don't know where you went to school, but when I took chemistry in the 7th grade one of the experiments was to burn strips of magnesium. Here is a video: https://www.youtube.com/watch?v=mFTY92VHYtc.

@TylerDurden in that respect schools worldwide should be on par. The point is that massive Mg part (think of a seat frame) will need a lot of heat to enter the molten state. The molten Mg though - that will act ...  with alacrity. As PeterKämpf points out, burn is not the problem with Mg. A lot of Mg parts are flying in helicopters, especially gear boxes, and they are not prone to spontaneous combustion, or we´d know about it. Actually, that is also what we teach our aerospace students.

Magnesium has initially been held back by technical difficulties in the removal of chloride inclusions. These had been solved only after a lot of progress had been made in aluminum alloys. Also, magnesium parts should only be used in a dry environment - when the metal is exposed to water, rapid corrosion must be expected.
The first application of magnesium in aerostructures was in the 1930s (mostly under the trade name Elektron), with a small boom in the 1950s. Lockheed even built one F-80C fully from magnesium. Magnesium was widely used in the former Soviet Union, but eventually phased out in newer designs due to corrosion problems. Flammability, by the way, was never a real concern with magnesium.
With improved protection techniques, magnesium might make a limited come-back. It is already used in castings and extruded parts. Per kilogram, it is twice as expensive than aluminum, but very easy to machine. However, at higher temperature it will lose strength even faster than aluminum.
Parallel to that, aluminum sheet can be made lighter and stiffer by adding up to 2.5% of lithium. These Al-Li alloys have found their way into the newest aircraft designs and will raise the bar for magnesium applications.
Magnesium and water really don't mix well: Trying to extinguish a magnesium fire with water is a memorable experience, as adding water will intensify the reaction. Use iron chips or sand instead to cool down the burning metal and to keep oxygen away. Historically, though, magnesium fires on aircraft have been extremely rare.

@JayCarr: When Wright used cast magnesium crankcases for their Duplex Cyclones, early B-29s had the misfortune of the magnesium burning through the wing spar when the engine caught fire. The flammability issue is real, but rare.

How much worse is a magnesium based alloy in a fire compared to aluminum or composites?

@brinnb: Magnesium is certainly worse, if only for the fact that firefighting it is not easy and needs very targeted means. Composites can be made almost self-extinguishing by adding the right flame retardants, but this reduces their strength. But aluminum will also burn if heated enough - it is best to avoid a fire altogether.

@PeterKämpf "it is best to avoid a fire altogether" I'm sure many many people agree with you, but that's a tall order

@raptortech97: A lot can be done - look at how carefully the Zeppelins were designed to avoid sparks or the collection of hydrogen-oxygen mixtures. Using magnesium instead of aluminum wouldn't had changed the fire risk.

A number of helicopters, including the Sikorsky H-34 had magnesium airframes.  They did have a reputation for buying like a Roman candle if hit by enemy fire.

One reason that the soviet Mg-alloys did not really spread outside of military aircraft applications was the occasional use of radioactive alloy components, namely Thorium. Regarding temperature, some Mg alloys beat e.g. 2024 at Tensile Strength beyond 200°C.


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What components of the Boeing 787 are made out of titanium?
Inspired by the question, Why is the 787 tail cone made out of titanium?, I want to know what other parts of a Boeing 787 are made out of titanium.
We now know, the APU exhaust cone is made out of titanium, but what else?

The Boeing 787 is special due to its high count of composite parts. When aluminium comes in contact with graphite fiber, contact corrosion will damage the aluminium whereas titanium will be unaffected due to its higher position in the electrochemical series.
15% of the initial 787s empty weight consists of titanium, a fraction much higher than in other airliners and one of the reasons that Boeing lost 23 million USD on each of their 787s sold initially.

787 material breakdown (picture source). Dark grey is carbon fiber composites and light grey is glassfiber composites.
The linked Reuters article contains a few parts which Boeing made from titanium in the first 787s:

Boeing has changed the cockpit window frame to aluminum from titanium, and has changed the frames of some doors to composite from titanium. Special coatings or materials are used to prevent aluminum corrosion, experts said. It wasn’t known how much Boeing saved by the change. Boeing declined to comment.

The article goes on to mention a part in the wing-fuselage connection which is milled from a solid block of titanium. 97.5% of the block is lost when it is machined - very similar to the Tornado center wing box!
Another use of titanium is in the hi-lok fasteners which are used to bolt metallic or graphite parts together. I would expect that those are the biggest use of titanium in regular aluminium airliners, because using titanium hi-loks instead of steel results in weight savings.

15%  !!!!!!!!! holy crap !!!!!!!!!


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How is lateral CG managed in aircraft with uneven seating?
On many smaller jets seating is unbalanced with 1 seat on one side and 2 on the other side of the aisle.
Some examples are the Embraer ERJ-145:
Source - SeatGuru
Embraer ERJ-135:
Source - SeatGuru
And Saab 340B:
Source - SeatGuru
Assuming a full flight (where passengers cannot be moved to make balance shifts), how is the lateral balance/center of gravity maintained?

@SMSvonderTann - Ah, searching didn't turn that up for me - wrong terms, I guess. Mention is made of that being a large aircraft, while I specifically refer to smaller aircraft. I presume the math is the same, is the resulting X inches equally small and, therefore, not an issue?

In all cases (MD80 and yours) only the central row is unbalanced, and is offset by 1/2 seat width, about 10"

Dang... my first duplicate question.


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Which type of control gives a more agile missile, tail or canard?
I read somewhere (sorry don't remember where) that canards controls for missiles are more effective than tail controls.  Is this true, and if so, why?

It is more effective, but is really more of a physics question regarding the force that can be applied to levers with different pivots.

Since the control unit sits at the tip of the missile, it is simply more practical to control it via canard deflections. The rear part can then be made as a simple pressure vessel with fixed fins and a nozzle.
If the rear fins would have to move, their actuators would either be in the way of the nozzle flow or would stick out on the outside - in both cases performance would suffer. Also, long cable runs along the side of the missile would need to be added.
By controlling the missile with the forward fins, any change of direction will be direct, whereas control via the rear fins means that the rear end of the missile would initially move away from the desired direction, until the change in attitude would move the missile towards its target. Thus, the canard will result in a more agile missile.


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Why are combi airliners no longer being built?
Why are combi airliners no longer being built?  Is the reason structural, economic or regulatory?

I guess it's economic: Now passenger volume is high enough to justify passenger-only aircraft. Also, previously only the big aircraft had the range for intercontinental routes, and passenger volume on thin routes was too low to justify a full DC-7C or DC-8 or Boeing 747 on them. Since smaller aircraft did not have the range, there were no aircraft available to serve those routes without adding cargo.

I may be misunderstanding but even in full passengers configuration airliners are able to transport cargo. Thus it may not be interesting to change the volume dedicated to passengers from flight to flight as it seems to be possible for combi, the airliners being able to transport both cargo and passengers.

@PeterKämpf -- there are still routes thin enough to justify combis, usually in remote areas.  I suspect the problems of firefighting in large class B bays have more to do with the lack of new-build combis than any economic reason -- otherwise we'd be seeing replacements soon for the gravelkitted B732 combis flown in the Canadian Arctic.  (They're getting close to their cycle life...)

This seems only makes sense when you need a fraction of airplane worth of cargo or passengers. e.g. you have one flight per day and you want it to be split this single flight half and half between cargo and passenger. Most airlines operates dozens of flight per day per destination and don't need to split on the single flight level. Likewise nobody need a half-burger-half-sub meal because most people can eat a burger at noon while a sub in the evening.

While combi configurations are rare, they do have a place -- for instance, when operating narrowbodies into remote locations which have a demand for cargo that exceeds what can be put in the belly, but also need pax transport.
The most recent combi configuration I can find a source for is the 2008 work done by Aviation Traders to configure Iron Maiden's touring plane.  In this case, they used the existing seat rails to mount pallet fittings in order to avoid structural changes to the floor, and also are taking special steps (such as firebagging of cargo) to mitigate against a main deck cargo compartment fire.
As to why they aren't made otherwise?  It clearly isn't operating economics, at least for Alaska Airlines, Canadian North, First Air, and other current combi operators -- otherwise they'd have retired their combis by now.  The main issue, I strongly suspect, is actually the unsolved problem of fire suppression in large, main deck class B cargo compartments, such as those found on combi aircraft.  Aviation Traders' work on Class F is intended to solve this problem, made famous by the crash of SAA 295 after its main deck class B bay experienced a fire of unknown origin that proved catastrophic.

Combi aircrafts are basically modified airliners that carry passengers and/or cargo.
Though there are multiple reasons for not combi aircrafts not in use nowadays, the main reason is economic.

In case of mixed passenger/cargo operations, the cargo is usually one directional. The aircraft will be half empty in the return flight.
The conversion of passenger aircrafts involve some significant structural changes like cutting open a cargo loading door.

Source: www.airtransport.cc

Air cargo is highly seasonal, while the passenger traffic is not (Except for some spikes during festive occasions). It makes little economic sense to operate combi aircraft while passenger and cargo aircrafts can be operated separately more economically.
The conversion of he aircraft to carry cargo is not one of simply removing seats. it involves modification/installation of cargo handling systems, fire extinguishing systems etc. For example, air cargo pallets are standardized and cannot be transferred through cabin doors. The figure shows the work involved in the conversion of a Boeing 757

Source: staero.aero

The airline passenger traffic has grown enormously through th years, while the air cargo has not seen such robust growth. So it makes sense to concentrate on only a single sector.

Source: centreforaviation.com
The only places combi aircraft are used are those in which operating such aircraft is necessary. Simply put, combi aircrafts belong to another age of air transportation, while in the present (and near future), it makes more economic sense to operate specialized passenger and freight aircraft.

I think the term is being slightly misused :)
A combi aircraft is not a type of aircraft to be built, but rather an aircraft that is simply configured to transport both passengers and freight.
Alaska is an example of an airline that uses aircraft configured in this fashion.
One reason they aren't more widespread is that they inhibit economies of scale. They are more difficult to use effectively - you need a special purpose for them.

Yes, but the configuration usually involves special fittings. Different floors designed to handle pallets, cargo doors, that sort of thing.

@egid True, a significant investment perhaps. 'simply' is probably a bit strong of a qualifier.

In addition to the above answers:

Tightened cargo security since September 11. Comparing to cargo-only aircraft,
the cargo on passenger aircraft or combi aircraft must be restricted and limited.
Airfreight charge was kept lower due to big competition in the market.
Passenger business might be more profitable in using aircraft space.


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How can the Airbus A380 have such a high lift/drag with a relatively low aspect ratio wing?
The Airbus A380 has a wing with aspect ratio between 7 and 8.  Reference here.  The lift to drag ratio is somewhere between 19 and 20, which is comparable to aircraft with higher aspect ratios and superior to aircraft with similar aspect ratios.  A reference to the lift/drag number is here.  How can the A380 achieve this?

The aspect ratio of the aircraft is usually kept as high as possible to reduce the induced drag. However, it forms only one part of the total drag acting one the body.
The L/D ratio can be improved by either increasing the (coefficient of) lift or decreasing the (coefficient of) drag.
While no data is available in open sources, it is likely that the wing airfoil was designed for high lift in cruise conditions. Also, the wing has a number of high-lift-devices to increase lift like the flaps, droop nose etc.
For example, in order to improve the L/D ratio during initial takeoff and climb, the A380 incorporated a droop nose device in the inboard leading edge. This improved the lift by around 5%. The figure below shows A380 deploying droops during landing

"A380 F-WWEA LEGT" by User:Keta - Own work. Licensed under CC BY-SA 2.5 via Commons.
The aerodynamic drag on a body is a combination of a large number of components like

Induced Drag
Parasitic Drag
Wave Drag.

The (lift) induced drag is lesser for aircrafts with higher aspect ratio. In designing the A380 however, the limitations of airport operations meant that the wingspan was limited. This resulted in a rather low aspect ratio of 7.8. However, the same effect(of higher aspect ratio) can be achieved using wingtip devices such as winglets. A380 uses wingtip fences to reduce induced drag.

"British Airways Airbus A380-841 F-WWSK PAS 2013 10 Wingtip device" by Julian Herzog. Licensed under CC BY 4.0 via Wikimedia Commons.
As the speed increases, the induced drag reduces and the parasitic drag (of which the form drag is a component) comes to dominate the drag produced.

"Drag Curve 2" by Original uploader was GRAHAMUK at en.wikipedia - Transferred from en.wikipedia; transferred to Commons by User:Sfan00_IMG using CommonsHelper.. Licensed under CC BY-SA 3.0 via Commons.
The major part of the parasitic drag is the form drag, caused by the movement of aircraft through the air. The A380 design reduces this by reducing the control surface areas through the use of Fly by wire system.
This is also achieved by reducing the drag caused refining the aerodynamics of the aircraft, such as by using improved flap track fairings and increased size of the belly fairing.

"Singapore Airlines A380 9V-SKH" by Simon_sees - Singapore Airlines A380 9V-SKH. Licensed under CC BY 2.0 via Wikimedia Commons.
The overall aircraft configuration and not just the individual components were analyzed using CFD, which led to further optimization of the aerodynamics design and reduction of drag by changing the spanwise distribution of camber and twist from root to tip (around 2%).
The airfoil used was most probably optimized for reducing wave (transonic) drag, though no data is available in open literature.
Even though all these improvements were only marginal individually, put together, they have a significant impact on the overall L/D ratio.
One more thing to note is that though the L/D ratio might appear high, the value is actually comparable (and even less than) the aircrafts that enter service after A380. So in a way, there is nothing surprising about the L/D ratio of A380 except that it illustrates the steady progress being made in civil airliner aerodynamic design.

Droop or Droop nose device is the term used for the leading edge high lift device in A380 inboard wing. They are not slats. In case of slats, a separate panel detaches itself from the wing leading edge and drops. In case of droops, the entire leading edge rotates forward. in case of slats, there is a gap (the slot) between the slat and the wing LE. there is no such thing in case of the droop

Do you have any idea what is the $V_Y$ (a.k.a. “green dot speed”)? Since not much can be done about increase of induced drag due to low aspect ratio and most of the aerodynamic refinements affect form drag, I would expect it to be somewhat higher, but I don't have any reference.

A benefit that the A380 has in terms of drag is its size.
As the aircraft is very large, the Reynolds numbers appearing on the wing are larger.
For a turbulent boundary layer on a hydraulically smooth plate, Schlichting$^{[1]}$ proposes the following approximate relation for the Turbulent friction coefficient, $C_f$:
$$ C_f=\frac{0.0472}{(log_{10} Re)^{2.58}} $$
This shows that for increasing the value of $Re$, the value of $C_f$ decreases.
As such, a larger wing will lead to a lower friction coefficient.
It should be noted that the total friction drag might still increase with increasing wing size.
[1] Schlichting, H., Gestern, K.: Boundary Layer Theory, 8th edn. Springer, Berlin (1999)


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Why are twin engine airliners more fuel efficient than three or four engine airliners?
It is said that given roughly similar aerodynamic and engine technology that twin engine airliners are more fuel efficient than 3 or 4 engine airliners.  Why is this?  The twin engine airliners tend to have more excess power so I'm thinking that they can climb faster and fly higher, lowering the amount of time spent climbing (uses more fuel than cruise) and lowering fuel consumption during cruise.  I suppose that fuel consumption is less while the engines are idling as well.  Are there other factors that go into the 2 vs 3 vs 4 engine fuel consumption issue?

Not a fully definitive answer, but a large part of the "efficiency" of an aircraft is drag and weight. Two (larger) round engines will have less surface causing drag than four smaller ones. In addition, each engine will add complexity to the aircraft (fire monitors, fuel piping, extra hydraulics) as well as necessitating duplication of things like thrust reverses, all of which add extra weight and sometimes drag.
So, all things being equal, one huge engine would be most "efficient". This would obviously be difficult, and impractical for most passenger jets, so when they will fit, without scraping the ground, two are used. If more thrust is needed, then they break it down into three or four.
(Note, the main reason 4 engines were used in the first place was for redundancy. At least one country's [US?] regulations stated that the aircraft must be capable of sustained flight, including climbing, with the loss of an engine. 2-1 = 50% thrust lost where as 4-1= only 25% loss, therefore requiring less reserve power)
*Edit note #2
"Twin engine airliners tend to have more excess power" they climb faster may be relevant in terms of efficiency, but part of the reason a twin will usually have more reserve thrust is due to the redundancy "keep flying" rules described above.

The original reason for using 3 or 4 engines was not the requirement to continue a climb after V1 cut (which is still a requirement for all large, transport-category multis,) but rather the legal requirement that all aircraft operating more than a certain distance away from an airport where they could reasonable land have at least 3 engines. That was back in the days when engines were much less reliable and failed a lot more often. ETOPS changed that.

@reirab, the very original reason was simply that the engines were not powerful enough, and one needed several of them to fly. Multi-engine aircraft from 1910s-30s commonly couldn't even fly level (in many practical conditions) with one engine failed. In fact, even 'modern' small GA twins (like PA44) have grave problems with one engine failing on takeoff.

@Zeus Yes, that's true. My comment was referring to jet airliners.

Part of the reason is that large engines tend to be thermodynamically more efficient than small ones, and this isn't specific to any one type of engine.
Theoretical engine cycle is understood in terms of compressing, heating, expanding volumes of gas in an ideal environment. The closer the engine approximates to that environment the closer the engine can get to its ideal efficiency.
Relatively cool metal surfaces like combustion chamber walls, or pistons and cylinder liners, detract from this, so the further you can keep the gas from the metal, (i.e. the larger the engine) the more efficient (for the same theoretical cycle).
Off-topic, large marine diesels can exceed 50% thermodynamic efficiency while small diesels may reach 40% as seen here. The same table shows this applies to gas turbines too, comparing a 36 MW gas turbine (Trent, 39.8%) to a 2.20MW one (P&W PW127, 27%) but these are just example numbers, not a really fair comparison. (It would be nice to see like-for-like-but-bigger comparisons)
So larger engines tend to have higher thermodynamic efficiency, it just so happens you also need fewer of them.

I'm not sure if the twin engined aicraft are more fuel effecient compared to their four engined counterparts. Is there any hard data available?
In general, newer aircraft are more fuel effecient compared to older ones. See http://www.transportenvironment.org/sites/te/files/media/2005-12_nlr_aviation_fuel_efficiency.pdf. For example, the Boeing 747-8 first flown in 2011 is more fuel efficient compared to the 787-8 which flew first in 2009 while being less efficient compared to 787-9 (which flew in 2013). It is instructive to note that the 747-8 uses the same engines as 787-8.
We can say the twinjets are more efficient because of the engines used in them. Almost all the aircraft under development are twinjets, with the A380 being the last 4 engine aircraft to enter (commercial) service. These aircraft incorporate a number of recent advances in the field (like increased use of composites along with better engines), which makes them more fuel efficient.
So, there is no reason to say that twinjets will be more efficient than four engines aircraft, but are usually so because of various other developments, which are also equally applicable to the four engined types.
Note: See the References given in https://en.wikipedia.org/wiki/Fuel_economy_in_aircraft for comparison of fuel effeciencies of aircrafts.

Actually, the 747-8 was the last large quad-jet to enter commercial service (in the West, at least... not sure about Russian/Chinese models.)

What sources say that the 747-8 is in between the 787-8 and 787-9 in efficiency? That doesn't seem right and the linked Wikipedia page does not agree.

Check the values for medium haul flights. Also, the values vary depending on the number of seats too. In long haul flights, there are two values for 747-8. The better one was with 467 seats, about which airbus complained. See http://aviationweek.com/commercial-aviation/airbus-boeing-fight-over-747-a380-performance-data. The other one is with 405 seats. See http://leehamnews.com/2014/02/03/updating-the-a380-the-prospect-of-a-neo-version-and-whats-involved/. See http://www.pprune.org/fragrant-harbour/492611-747-8-fuel-burn.html for discussion on 747-8 fuel burn.

The airlines were prepared to accept harsh extra regulations in exchange for being allowed to use twin engine planes on transoceanic routes. This suggests that the advantage of twins is very real.